The invention relates to a method for multiple carrier modulation of digitally coded data wherein a time sequence of OFDM symbols is generated which are patterned into successive OFDM transmission frames, each OFDM transmission frame including a frame head with one or several control symbols and a guard interval between successive control symbols, and to a method for multiple carrier demodulation of thus modulated signals. Methods of this type are known from FR-A-2 639 495.
In the known multiple carrier modulation (orthogonal frequency division multiplex modulation, abbreviated OFDM modulation) according to FR-A-2 639 495, a time sequence of OFDM symbols is generated by Fourier synthesis of many carriers modulated with digitally coded data. The OFDM symbols are patterned into successive OFDM transmission frames which are separated from one another, for example, by a zero symbol (FIG. 1). Each OFDM transmission frame is comprised of a frame head with one or several control symbols and a subsequent useful data range with a multiplicity of data symbols. In the OFDM decoder, the control symbols serve to recognize the beginning of each received OFDM transmission frame and OFDM symbol at the correct time as well as to recover the exact modulation carrier frequencies according to amount and phase. On the modulator-end, a guard interval is respectively provided between successive OFDM symbols (control and data symbols). Because of the guard interval, a crosstalk of successive OFDM symbols stemming from a multipath propagation can be prevented on the demodulator-end. Here, the time length of the guard intervals for all OFDM symbols in the frame head and in the useful data range of each OFDM transmission frame is identical.
It turned out, however, that in the event of great delay time differences at a receiving location, particularly in common-wave networks, the guard intervals must be designed to have a relatively long duration so as to reliably prevent crosstalk between successive OFDM symbols. As a result of such a long duration of the guard intervals, the useful signal capacity or the transmission efficiency is reduced. One option for remedying the situation might be that, along with an extension of the length of the guard intervals, the time length of the useful intervals is extended to the same degree. This, however, increases the expenditure on the demodulator end considerably. Namely the demands with respect to the precision of the scanning of the OFDM data symbols, to the volume of the scanned values to be stored as well as to the computing expenditure for the frequency analysis of the time signal given by the scanned values are increased in excess proportion. Therefore, a selected guard interval for an OFDM modulation system represents a compromise in which drawbacks with regard to the transmission capacity, the receiver expenditure and the frequency efficiency must be accepted from a network planning point of view.